Thin film transistor, method of fabricating the same, and flat panel display using thin film transistor

ABSTRACT

A thin film transistor may include an active layer formed on an insulating substrate and formed with source/drain regions and a channel region; a gate insulating film formed on the active layer; and a gate electrode formed on the gate insulating film. The gate electrode may be formed of a conductive metal film pattern and a conductive oxide film covering the conductive metal film pattern. The source/drain regions may include an LDD region, and the LDD region may at least partially overlap with the gate electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korea Patent Application No.2003-84241 filed on Nov. 25, 2003, disclosure of which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor (TFT), a methodof fabricating the same and a flat panel display using the TFT. Moreparticularly, the invention relates to a TFT having a gate overlappedlightly doped drain (GOLDD) structure, a method of fabricating the sameand a flat panel display using the TFT.

2. Description of the Related Art

An active-matrix flat panel display can use a TFT as a switchingelement. TFTs may be used, for example, as pixel-driving TFTs formed ineach pixel and driving the pixels, and as driving-circuit TFTs drivingthe pixel-driving TFTs and transmitting a signal to a scan line (gateline) and a signal line (data line).

Among various TFTs, a polycrystalline silicon TFT can be fabricated at atemperature similar to that for fabricating an amorphous silicon TFT dueto technological advances. In particular the advance of crystallizationtechnology using lasers may permit this relatively low temperaturefabrication of TFTs. Polycrystalline silicon may allow electrons orholes to have high mobility as compared with amorphous silicon in a TFT.It thus may be possible to realize an improved complementary metal-oxidesemiconductor (CMOS) TFT having n- and p-channels. Accordingly,polycrystalline silicon can be used to form pixel-driving TFTs anddriving-circuit TFTs on large-sized insulating substrates.

In a polycrystalline silicon CMOS TFT, an n-channel metal oxidesemiconductor (NMOS) TFT generally uses phosphorus (P) as a dopant.Phosphorus (P) has an atomic weight greater than boron (B): the dopantgenerally used in a p-channel metal oxide semiconductor (PMOS) TFT. Onereason for this choice is so that a silicon crystal lattice thereof isless likely to become damaged at a predetermined region and remainunrecovered in a sequential activating process.

Such damaged region decreases the mobility of electrons. This decreasein electron flow into a gate insulating film or a metal-oxidesemiconductor (MOS) interface, when the electron is accelerated from asource region to a drain region, is called hot carrier stress.Therefore, the damaged region may have a deleterious effect on circuitoperation of the flat display panel, and may increase the off-current.

To solve the foregoing problems, various structures such as an off-setstructure, a lightly doped drain (LDD) structure, and the like have beenproposed. In the case of the off-set structure, an off-set region may beprovided to form an imperfect doping region on a predetermined regionbetween the gate and the source/drain regions, so that an electric fieldapplied to a junction area is reduced by great resistance due to theoff-set region, thereby decreasing the off-current. In the case of theLDD structure, an LDD is formed by lowering the doping concentrationapplied to a predetermined region between the source and drain regions,so that the off-current is decreased and the reduction of the on-currentis minimized.

However, as low temperature poly silicon (LTPS) technology is highlyintegrated, the conventional off-set and LDD structures provide limitedenhancement to the reliability of a short channel device. One way toovercome that limit may be to implement a thin film transistor with agate overlapped lightly doped drain (GOLDD) structure.

FIGS. 1A, 1B, 1C, and 1D are cross-sectional views for illustrating afabrication process of a conventional thin film transistor with a GOLDDstructure.

As shown in FIG. 1A, a buffer layer 110 may be formed on an insulatingsubstrate 100. Then an amorphous silicon film may be deposited on thebuffer layer 110 and crystallized into a polycrystalline silicon film.Thereafter, an active layer 120 may be formed by patterning thepolycrystalline silicon film.

After forming the active layer 120, a gate insulating film 130 may beformed on a substantial portion of an entire surface of the insulatingsubstrate 100 formed with the active layer 120.

After forming the gate insulating film 130, a first photoresist pattern140 may be formed for doping low-concentration impurities having apredetermined conductive type (i.e., for LDD doping).

After the first photoresist pattern 140 is formed, the low-concentrationimpurities may be doped using the first photoresist pattern 140 as amask, so that low-concentration source/drain regions 123S, 123D may beformed on the active layer 120. A region between the low-concentrationsource/drain regions 123S and 123D may be used as a channel region 121of the TFT.

As shown in FIG. 1B, after forming the low-concentration source/drainregions 123S, 123D on the active layer 120 through the light doping, thefirst photoresist pattern 140 may be removed, and a gate electrodematerial film 150 may be formed on the gate insulating film 130. Then, asecond photoresist pattern 160 is formed for forming a gate electrode.

Here, the second photoresist pattern 160 is formed, partiallyoverlapping with the low-concentration source/drain regions 123S and123D. Further, the width of the overlapped region is limited to aminimum of about 0.5 μm or more depending on resolution of a stepper.

As shown in FIG. 1C, a gate electrode 155 may be formed by patterningthe gate electrode material film 150 using the second photoresistpattern 160 as the mask. In this situation, the gate electrode 155 maybe formed partially overlapping the respective low-concentrationsource/drain regions 123S and 123D due to the second photoresist pattern160.

After forming the gate electrode 155 to overlap with the respectivelow-concentration source/drain regions 123S and 123D, high-concentrationimpurities may be doped onto the active layer 120 through the gateelectrode 155 used as the mask, thereby forming high-concentrationsource/drain regions 125S and 125D.

As shown in FIG. 1D, an interlayer insulating film 170 having contactholes 171, 175 through which the high-concentration source/drain regions125S, 125D are partially exposed is formed on the entire surface of theinsulating substrate 100 with the gate electrode 155. Then, source/drainelectrodes 181, 185 are formed to be electrically connected to thehigh-concentration source/drain regions 125S, 125D through the contactholes 171, 175, thereby finally forming a thin film transistor with theGOLDD structure.

However, in a conventional thin film transistor with GOLDD structure, itmay be difficult to reduce the low-concentration source/drain regionsoverlapping with the gate electrode. That is, it may be difficult toreduce the width of an LDD range to about 0.5 μm or less because of theresolution of the stepper used in the fabrication process.

Further, in a conventional thin film transistor with GOLDD structure,the low-concentration impurities may be doped using a photoresist mask.Then, after the gate electrode is formed, high-concentration impuritiesmay be doped. Thus, an additional mask is needed to dope thelow-concentration impurities. Another problem that can occur is that thegate electrode can be defectively aligned.

SUMMARY OF THE INVENTION

The present invention provides a thin film transistor with a GOLDDstructure, a method of fabricating the same, and a flat panel displayusing the same. In an embodiment of the present invention the gateelectrode may be formed of a conductive metal film pattern and aconductive oxide film may be formed by oxidizing the conductive metalfilm pattern. Thus, the width of an LDD region can be easily adjustedand defective alignment of the gate electrode can be prevented.

The present invention separately provides a thin film transistorincluding an active layer formed on an insulating substrate and havingsource/drain regions and a channel is region. The TFT also may include agate insulating film formed on the active layer and a gate electrodeformed on the gate insulating film. The gate electrode may be formed ofa conductive metal film pattern and a conductive oxide film covering theconductive metal film pattern. In such a TFT the source/drain regionsmay have LDD regions, and the LDD regions may overlap with the gateelectrode.

The present invention also provides a method of fabricating a thin filmtransistor. This fabrication may include the following steps: forming anactive layer on an insulating substrate, forming a gate insulating filmon the active layer, forming a conductive metal film pattern on the gateinsulating film, performing light doping on the active layer (using theconductive metal film pattern as a mask), forming a gate electrode thatincludes a conductive metal film pattern and a conductive oxide filmformed by oxidizing the conductive metal film pattern and covering theconductive metal film pattern; and forming source/drain regions. Thestep of forming source/drain regions may be accomplished by highlydoping the active layer using the gate electrode as a mask. Thesource/drain regions may have LDD regions, and the LDD regions mayoverlap with the gate electrode.

The present invention also provides an active matrix flat panel displayor an active matrix organic electroluminescence display, which uses athin film transistor of the type described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D are cross-sectional views for illustrating thefabrication process of a conventional thin film transistor with a GOLDDstructure.

FIGS. 2A, 2B, and 2C are cross-sectional views for illustrating afabrication process of a thin film transistor with a GOLDD structureaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout thespecification.

FIGS. 2A to 2C are cross-sectional views for illustrating a fabricationprocess of a thin film transistor with a GOLDD structure according to anembodiment of the present invention.

A thin film transistor with a GOLDD structure according to an embodimentof the present invention may have a structure in which a gate electrode240 may be formed of a conductive metal film pattern 241 and aconductive oxide film 245 covering the conductive metal film pattern241. The gate electrode 240 may overlap with an LDD region that servesas a low-concentration doping region provided in an active layer 220.

As shown in FIG. 2A, a buffer layer (diffusion barrier) 210 may beformed on an insulating substrate 200 by plasma enhanced chemical vapordeposition (PECVD), low-pressure chemical vapor deposition (LPCVD),sputtering, or the like. Such a buffer layer may provide the beneficialeffect of preventing impurities such as, for example, metal ions, frombeing diffused into or otherwise infiltrating the active layer(amorphous silicon).

After forming the buffer layer 210, an amorphous silicon film may bedeposited on the buffer layer 210 by PECVD, LPCVD, sputtering method, orthe like. Then, a dehydrogenation process may be performed in a vacuumfurnace. In the case in which the amorphous silicon film is deposited bysputtering method, the dehydrogenation process may be omitted.

Thereafter, a crystallization process of applying high energy to theamorphous silicon film may be performed to crystallize the amorphoussilicon, thereby forming a polycrystalline silicon film. Preferably,excimer laser annealing (ELA), metal induced crystallization (MIC),metal induced lateral crystallization (MILC), sequential lateralsolidification (SLS), solid phase crystallization (SPC), or the like canbe used as the crystallization process.

After forming the polycrystalline silicon film, an active layer 220 maybe formed by patterning the polycrystalline silicon film.

Thereafter, a gate insulating film 230 may be deposited on the activelayer 220, and a conductive metal film may be deposited on the gateinsulating film 230. Then, a conductive metal film pattern 241 may beformed by patterning the conductive metal film.

Here, the conductive metal film pattern 241 may preferably be made of amaterial which itself is conductive and whose oxide is also conductive.Particularly, the metal film pattern 241 may be made of Zn, In or analloy of them. Thus, the conductive oxide film together with theconductive metal film pattern can function as the gate electrode.

After forming the conductive metal film pattern 241, impurities having apredetermined conductive type are lightly doped using the conductivemetal film pattern 241 as a mask, that is, a lightly doped drain (LDD)doping is performed using the conductive metal film pattern 241 as themask, so that low-concentration source/drain regions 223S, 223D areformed. At this time, a region between the low-concentrationsource/drain regions 223S and 223D forms a channel region 221 of thethin film transistor.

As shown in FIG. 2B after forming the low-concentration source/drainregions 223S, 223D, the conductive metal film pattern 241 is oxidized bya heat treating process. At this time, an exposed portion of theconductive metal film pattern 241 is oxidized and forms the conductiveoxide film 245 while being oxidized, thereby forming the gate electrode240 with the conductive metal film pattern 241 and the conductive oxidefilm 245.

As the conductive metal film pattern 241 is oxidized, two things mayoccur. The metal film pattern 241 may shrink a little as conductivemetal reacts with oxygen. However, the thickness of the conductive oxidefilm 245 may increase more than the metal film pattern 241 decreases.Thereby the width of the resultant gate electrode 240 may be more thanthe width of the conductive metal film pattern 241. Hence, the gateelectrode 240 may partially overlap with the low-concentrationsource/drain regions 223S and 223D.

The conductive oxide film 245 formed at the sides of the conductivemetal film pattern 241 preferably may have a thickness of about 2 μm orless, and more preferably of about 1 μm or less. The conductive oxidefilm 245 together with the conductive metal film pattern 241 may be usedas a mask for a subsequent high-concentration doping. These two layersmay form the GOLDD structure, thereby determining the low-concentrationsource/drain regions 223S, 223D. That is, they may control the width ofthe LDD region.

The thickness of the conductive oxide film 245 may vary depending uponheat treating conditions for the conductive metal film pattern 241, andthus the LDD region (223S, 223D) can have a width of about 2 μm or lessin proportion to the thickness of the conductive oxide film 245. In somepreferable embodiments, the LDD region (223S, 223D) can have a width ofabout 1 μm or less.

After the gate electrode 240 is completed (including both the conductivemetal film pattern 241 and the conductive oxide film 245), the highdoping may be performed on the active layer 220 using the gate electrode240 as a mask. This doping may thereby form the high-concentrationsource/drain regions 225S and 225D.

Next, the low-concentration source/drain regions 223S and 223D formedunder the conductive oxide film 245 formed at the sides of theconductive metal film pattern 241 may be shielded from being highlydoped because of the gate electrode 240. Thus the low-concentrationsource/drain regions 223S and 223D may remain in the low-concentrationdoped state and may function as an LDD region. Thus, the gate electrode240 may overlap with the lightly doped region 223S and 223D (that is, itmay overlap with the LDD region), thereby forming the GOLDD structure.Here, the LDD region may be formed beneath the conductive oxide film 245formed at the sides of the conductive metal film pattern 241.

Further, the width of the LDD region of the GOLDD structure may bedetermined by the thickness of the conductive oxide film 245 formed atthe sides of the conductive metal film pattern 241, so that the width ofthe LDD region formed overlapping with the gate electrode 240 may benarrower than the thickness of the conductive oxide film 245 formed atthe sides of the conductive metal film pattern 241.

As shown in FIG. 2C, after the high-concentration source/drain regions225S and 225D are formed, an interlayer insulating film 250 may beformed on the entire surface of the insulating substrate 200 and may bepatterned to include contact holes 251 and 255 through which thehigh-concentration source/drain regions 225S and 225D may be partiallyexposed.

After the contact holes 251 and 255 are formed, a predeterminedconductive film may be deposited on the entire surface of the insulatingsubstrate 200 and may be patterned to form source/drain electrodes 261and 265 that may be electrically connected to the high-concentrationsource/drain regions 225S and 225D. This may complete the formation of athin film transistor with the GOLDD structure.

Such a thin film transistor does not require an additional mask for thelow-concentration doping. Moreover, in addition to reducing the numberof required masks, the manufacturing technique described may alsoprevent the gate electrode 240 from being defectively aligned.

Also, the GOLDD structure may be formed by means of the conductive oxidefilm 245 formed by oxidizing the conductive metal film pattern 241. Thusthe width of the LDD region can be adjusted by the thickness of theconductive oxide film 245 formed at the sides of the conductive metalfilm pattern 241. Hence, it may be relatively easy to adjust the widthof the LDD region. It may even be possible to form the LDD region havinga width of about 2 μm or less, or better yet, a width of about 1 μm orless.

Further, a method of fabricating an active matrix flat panel displaysuch as an active matrix liquid crystal display (LCD) and an activematrix organic electroluminescent display can be implemented using athin film transistor with the foregoing GOLDD structure. Thus, thistransistor may aid in providing an active matrix flat panel display.

As described above, the present invention discloses a thin filmtransistor with a GOLDD structure, a method of fabricating the same, anda flat panel display using the same. In these embodiments, a gateelectrode may be formed of a conductive metal film pattern and aconductive oxide film. The conductive oxide film may be formed byoxidizing the conductive metal film pattern. This may result in aneasily adjustable width of the LDD region. Additionally it may help toprevent defective alignment of the gate electrode.

While the present invention has been described with reference toparticular embodiments, it must be understood that the disclosure hasbeen made for purpose of illustrating the invention by way of examplesand is not limited to limit the scope of the invention. One ordinarilyskilled in the art can make changes to the disclosed embodiments withoutdeparting from the scope of the invention.

1. A thin film transistor, comprising: an active layer formed on aninsulating substrate and having source/drain regions and a channelregion; a gate insulating film formed on the active layer; and a gateelectrode formed on the gate insulating film, and comprising aconductive film pattern and a conductive oxide film covering an uppersurface of the conductive film pattern, wherein the source/drain regionscomprise a lightly doped drain (LDD) region, and the LDD region overlapswith the gate electrode, wherein the conductive film pattern comprisesat least one of a group of Zn, In, and any alloy thereof.
 2. The thinfilm transistor of claim 1, wherein the conductive oxide film is formedby oxidizing the conductive film pattern.
 3. The thin film transistor ofclaim 1, wherein the conductive oxide film is about 2 μm or less thick.4. The thin film transistor of claim 1, wherein the conductive oxidefilm is about 1 μm or less thick.
 5. The thin film transistor of claim1, wherein the LDD region extends horizontally beneath a portion of theconductive oxide film formed at a side of the conductive film pattern.6. The thin film transistor of claim 1, wherein the LDD region isnarrower than the thickness of the conductive oxide film.
 7. The thinfilm transistor of claim 1, wherein the LDD region is about 2 μm or lesswide.
 8. The thin film transistor of claim 1, wherein the LDD region isabout 1 μm or less wide.
 9. An active matrix flat panel displaycomprising a plurality of thin film transistors of claim 1 arranged tocontrol the operation of a matrix of pixels.
 10. The active matrix flatpanel display of claim 9, wherein the flat panel display is a liquidcrystal display or an organic electroluminescent display.
 11. The thinfilm transistor of claim 1, wherein the conductive oxide film covers theupper surface of the conductive film pattern and a side surface of theconductive film pattern.